Unless the car you're working on has only one prior paint job, and it's still in good shape, you should figure on stripping the surface to metal. This may sound excessive, but again, the cars we usually deal with have been around a long time, and very few of them are blessed with original paint. Several re-sprays later, there's no telling what sins have been concealed. Even if the body itself is free of damage and filler but has the remnants of a few paint jobs, it's best to get them all off. Any problems with any of the prior finishes can manifest itself in your new paint job later on, and trust us, you only want to do this once. Removing that paint can be accomplished in several ways, from brutally basic sanding to high-tech chemical stripping. Consider your options:
This is the most basic means of removing old paint from your car, but it's also the most tedious and time consuming. However, if the current finish on your car is thin or peeling, it may not take much effort with a sander to get it down to bare steel. If you go this route, you'll probably be using a dual-action or "DA" sander. These are primarily air-powered and can be used for a variety of tasks during the refinishing process, so they're usually worth the investment. The dual action is the spinning of the sanding disc coupled with the spinning of the base that the sanding disc is mounted to. This results in an orbital motion that is desired because it tends to remove material more evenly than a simple spinning motion.
Most DA sanders can be switched to a single spinning motion, considered grinder mode, though this is generally reserved for heavy, localized material removal. Using a grinding action on the painted surfaces can create gouges in the finish, which is less of an issue when stripping to the bare steel as opposed to prepping a painted surface. However, grinding with aggressive grit, as you would when paint-stripping, can actually mar the steel, creating minor waves that can show up later. For the same reason, heavy-duty grinders, especially high-torque electric units, should not be used by the novice to strip paint. These machines have the ability to grind right into the sheetmetal, leaving scars that will have to be filled later. An 8-inch sanding disc with 80-grit paper on a DA should do the trick. The proper sandpaper for a DA usually comes in rolls of pre-cut paper discs with adhesive backing. You'll need a bunch to do the whole car.
This approach entails applying a chemical to the surface, which then softens and lifts the paint. After the product has done its thing, the bubbled paint must be scraped off of the surface and discarded. Some of the strippers are thick and must be brushed on while others can be sprayed, either from aerosol cans or pump bottles. The best approach is to strip in small sections, as there is usually a time window when the paint is fully softened. If you wait too long, the bubbled paint will begin to harden again, making it more difficult to scrape off. Like sanding, chemical stripping will take time, and cars with multiple paint jobs will require multiple applications before bare metal is reached. Some local governments consider the scraped paint to be a hazardous material, so proper collection will be expected, rather than simply throwing the remains in the trash. Some strippers are safe to use around rubber and chrome trim while others can damage these items, so be sure you know what you're dealing with before you begin. Also, some stripping products are intended for specific types of paint, like enamel, lacquer, and so on, so shop wisely. There are even some strippers that are designed to remove the paint but leave the primer, but if your car has been through multiple paint jobs, your best bet is to go to metal.
If chemical paint-stripping sounds like a good idea for your car but you'd rather remove yourself from the process, consider chemical dipping. As the term implies, the entire body of the car will be dipped in a vat of chemicals to remove all of the finishes, leaving it in bare steel. This service is provided by businesses that specialize in this type of work, and these places usually don't do any of the work to prepare the car for dipping, like removing everything that isn't the body itself. You'll have to handle that yourself, and you'll need to remove everything from the body prior to dipping, including the glass, brightwork, and all interior trim. In fact, even the remaining weatherstripping, seam-sealer, and undercoating will be dissolved. Some car-builders have concerns about chemical dipping, feeling that it can leave inaccessible areas--like the insides of the rocker panels--untreated and vulnerable to future rusting. However, dipping facilities that are equipped for treating car bodies usually pass the shell through multiple vats, including one that should leave an etched coating on the steel, protecting it from moisture. Despite this, you should be prepared to work on the body immediately after it is returned. Leaving the bare body outside after stripping is obviously out of the question.
Dipping is an excellent means of stripping, but it isn't right for every job. Obviously, if you didn't intend to completely dismantle your car and replace all of the weatherstripping, window seals, body seam sealer, and so on, this isn't the option for you. However, if you do select this method, consult with the stripping facility first to find out what they plan to do, and what they expect you to do before dropping the body off. Some dippers will reject a body that's too greasy or has excessive loose rust, as this can contaminate the dip.
One of the more popular means of having a car stripped is blasting--the process of using compressed air to shoot media particles at the body to abrade the finish. The most common form of pressure-blasting uses sand. However, sandblasting is not recommended for sheetmetal auto bodies, as the sand can be too aggressive. Even when fine sand is used, there is still an issue of panel warpage, since the abrasion quickly builds heat that can distort the steel.
A better method of blasting sheetmetal involves using plastic media. The small plastic particles usually have sharp edges that are very effective at stripping paint, yet the plastic won't create heat when it contacts the steel surface, so warpage isn't an issue. Typically, a car body will be completely dismantled prior to blasting, though it isn't absolutely necessary, as it is with dipping. Glass and other trim can be covered for media-blasting. Unfortunately, this media does end up in virtually inaccessible places, so this is certainly a concern.
Another emerging trend in body blasting is the use of baking soda as the media. The baking soda is also effective at stripping when applied under pressure, though it is pressurized with water rather than air. This process, considered wet pressure blasting, does not create dust and is not harmful to glass and trim. As a bonus, disposing of the baking soda can be as simple as washing it down the sewer, as some cities actually appreciate the addition of the baking soda to sewage since it will neutralize acids. Like media-blasting, baking soda blasting is a professional service, not a DIY deal.